The invention relates to a fuel cell assembly which uses fuel cells arranged in a stack between end plates.
Fuel cell assemblies, especially assemblies of molten carbonate fuel cells, where a number of fuel cells, which each contain an anode, a cathode and a porous electrolyte matrix arranged between them, are arranged in the form of a fuel cell stack.
In molten carbonate fuel cells, mixtures of alkali carbonates are used as electrolyte, causing the fuel cells to be liquid at the operating temperature. The electrolyte is contained both in the porous electrolyte matrixes and in the anodes and cathodes of the fuel cells, which are likewise made of porous material, and is kept there with capillary force. The function and efficiency of a molten carbonate fuel cell are dependent upon the complete and correct filling of the electrolyte, which is accomplished during manufacturing by adhering to tight tolerance settings. Both over-filling and under-filling with electrolyte negatively influence the efficiency and durability of the cells.
During fuel cell operation, parts of the electrolyte contained in the cells are lost due to various mechanisms, and primarily the following mechanisms:                due to the strong wetting property of the molten alkali carbonates, the electrolyte has the tendency of creeping out of the cell in the fringe area and onto the orifices that are provided for supplying and removing fuel gas and oxidation gas, wherein it then spreads to the exterior surface of the fuel cell stack and the adjacent components;        the alkali carbonates of the electrolyte enter into chemical reactions with construction materials of the fuel cells, wherein a portion of the electrolyte is bonded with the resulting chemical compounds; and        constituents of the alkali carbonates bond with water, which is created in the fuel cells as a reaction product, to form hydroxides, which evaporate at the operating temperature of the fuel cells.        
The gradual electrolyte loss during the life of the fuel cell leads to a decrease in power and possibly limits the life of the fuel cell.
One possibility for overcoming the above-mentioned difficulties is to provide an electrolyte reservoir to compensate the electrolyte losses from the fuel cells.
For example, German Patent DE 195 45 658 A1 has a molten carbonate fuel cell where a porous body with an electrolyte supply is provided in at least one place to compensate electrolyte losses. This porous body forming the electrolyte supply is assigned to the individual fuel cell; in a fuel cell assembly having a number of fuel cells arranged in the form of a stack. Thus each individual fuel cell would be provided with such a porous body for maintaining a supply of electrolyte.
Japanese Patent JP 61074265 A uses a matrix configuration for a fuel cell where the electrolyte is being distributed in the matrix from an electrolyte reservoir that is assigned to the matrix in order to compensate losses. Here as well, in the case of a fuel cell configuration having a number of fuel cells that are arranged in the form of a stack, each matrix of the individual fuel cells would be equipped with such an electrolyte reservoir.
Further suggestions in which each individual fuel cell should be equipped with electrolyte reservoirs for compensating electrolyte losses are disclosed in U.S. Pat. No. 5,468,573, U.S. Pat. No. 4,185,145, U.S. Pat. No. 4,548,877 and Japanese Patent JP 61277169 A.
Furthermore U.S. Pat. No. 4,467,019 and JP 07326374 A have fuel cell assemblies with several fuel cells that are arranged in the form of a stack, where the electrolyte matrix of each fuel cell, respectively, is connected with an electrolyte reservoir that is provided outside the fuel cell stack for the purpose of compensating electrolyte losses that occur.
Additionally, U.S. Pat. No. 4,761,348 has a fuel cell assembly where the ends of the fuel cell stack, have respective electrolyte reservoirs—one with an excess of electrolyte and one with a lack of electrolyte—which are separated from the complete cells of the stack by impermeable, yet electrically conductive separators, but are subjected to an electrolyte exchange with the fuel cells.
The existing solution suggestions have many disadvantages. In the case of individual electrolyte reservoirs that are provided in each fuel cell, only a limited amount of electrolyte can be maintained without a considerable increase in volume and cost of the cells. In the case of devices for filling the electrolyte supply in the individual cells, it is very difficult to distribute the replenish quantity exactly among the individual cells within the stack and fill each individual cell correctly. Channels or lines for filling the electrolyte form paths for parasitic currents along the fuel cell stack which currents reduce the power of the fuel cell assembly and possibly destroy the fuel cell assembly.
Another difficulty in connection with the loss and replenishing of electrolyte for fuel cells that are arranged in a stack consists of the fact that the electrically charged particles of the electrolyte migrate in the direction of the opposite polarity under the influence of the electric field that is generated by the fuel cell tension along the stack. The alkali ions contained in the electrolyte therefore have the tendency of migrating from the positive end to the negative end of the fuel cell stack under the influence of the electric field. Thus, the rate of electrolyte loss in the cells on the positive end of the fuel cell stack is considerably higher than that of the cells on the opposite end. Constantly maintaining or replenishing electrolyte for all cells would then lead to overfilling the cells in the vicinity of the negative end of the fuel cell stack and insufficient filling on the positive end.
It is therefore an object of the present invention to create an improved fuel cell assembly of the type having an electrolyte reservoir.
The invention creates a fuel cell assembly having a number of fuel cells that are arranged in the form of a stack, wherein each cell contains electrodes in the form of an anode and a cathode and a porous electrolyte matrix arranged between them, as well as a current collector for contacting the electrodes, and wherein furthermore an electrolyte reservoir for compensating electrolyte losses from the fuel cells is provided. Pursuant to the invention, the electrolyte reservoir is arranged on or in the vicinity of an end of the fuel cell stack, wherein the electrolyte is transported to the individual fuel cells by electrical forces within the fuel cell stack, and wherein hollow chambers, which are formed by a supporting structure and which contain porous bodies absorbing the electrolyte in the pores, serves as the electrolyte reservoir.
A considerable advantage of the fuel cell assembly pursuant to the invention is that electrical forces acting in the fuel cell stack cause the electrolyte to be automatically supplied to various positions within the stack while being adapted to the different electrolyte loss rates. Another benefit is that the invented fuel cell assembly is easy and inexpensive to manufacture and easy to operate. Another advantage is the elimination of leakage currents. This elimination is possible due to the lack of lines or channels along the fuel cell stack for distributing the electrolyte from the outside among the individual fuel cells paths. Since the electrolyte reservoir contains a supporting structure, the material which absorbs the electrolyte directly is not required to assume the supporting function. The appropriate material is therefore mechanically relieved, which is beneficial with regard to its creep stability.
The electrolyte is preferably one component of a spreadable or flowing paste, which is introduced into the hollow chambers of the structure, wherein additional components of the paste, after curing, create a porous body whose pores contain the electrolyte. The supporting structure could be, for example, a current collector, which is installed on the positive end (in fuel cells the cathode) between the end plate and the last cell. Similarly, a large-pored foam structure can be provided as the supporting structure, where the pores are filled with paste. Alternatively the paste can be introduced into recesses or bore holes of the end plate so that the end plate itself serves as the supporting structure of the electrolyte reservoir.
Pursuant to another beneficial aspect of the invented fuel cell assembly the electrolyte reservoir is installed on one end of the fuel cell stack and an electrolyte-absorbing reservoir, in the form of a porous body for absorbing excess electrolyte, is provided on the other end of the fuel cell stack. Accordingly, due to the migration of electrolyte from the electrolyte reservoir to the other end of the fuel cell stack, any excess electrolyte is removed over time. The porous body for absorbing excess material can thus be designed the same as the electrolyte reservoir, with the corresponding benefits.
The electrolyte reservoir is preferably installed on the positive end of the fuel cell stack, and the electrolyte-absorbing reservoir for absorbing excess electrolyte is provided on the negative end of the fuel cell stack.
Pursuant to a beneficial development of the invented fuel cell assembly the electrolyte reservoir can be filled. Electrolyte losses occurring during operation of the fuel cell can thus be compensated so that a continuously optimal operation of the fuel cell assembly is feasible.
Preferably an electrolyte filling line, which is connected with the electrolyte reservoir and extends from the fuel cell stack to the outside, for filling the electrolyte reservoir from the outside is provided.
A preferred embodiment provides for the electrolyte filling line to have a vertical or outwardly ascending course.
Pursuant to a particularly beneficial embodiment of the invented fuel cell assembly, the electrolyte filling line is provided for filling the electrolyte, which exists in solid form at ambient temperature, preferably in the form of pellets, wherein the solid electrolyte at the operating temperature melts in the fuel cell stack and is received by the electrolyte reservoir.
The electrolyte reservoir consists of a porous body, whose pores are filled with the electrolyte. The pore size of the electrolyte reservoir is preferably larger than that of the pores of the electrolyte matrix. Therefore, capillary forces support the transport of electrolyte from the reservoir to the matrixes of the fuel cells.
Pursuant to a preferred embodiment of the invented fuel cell assembly, the porous body of the electrolyte reservoir consists of fuel cell cathode material that is completely impregnated with electrolyte.
Pursuant to another preferred embodiment of the invented fuel cell assembly, the supporting structure of the electrolyte reservoir consists of an electrically conductive material, which serves as the electrical connection between the last fuel cell and the end of the fuel cell stack.
Pursuant to a further beneficial embodiment of the invented fuel cell assembly it is provided that along the fuel cell stack between individual components of the fuel cells and/or the fuel cell stack existing capillary travel paths for the electrolyte are designed with regard to their thickness and/or their pore size such that an optimization of the electrolyte transport within the fuel cell stack from the electrolyte reservoir to the fuel cells takes place. Therefore the speed of transport and the type of distribution of electrolyte delivered from the electrolyte reservoir to the individual fuel cells can be optimized.
Pursuant to another preferred embodiment of the invented fuel cell assembly, means for monitoring the tension of the most positive fuel cell or a group of most positive fuel cells are provided and a decrease in this tension is used a signal for filling the electrolyte supply in the electrolyte reservoir. Because of the electrical forces within the fuel cell stack, the electrolyte loss of the fuel cells increases as forces are on the positive end of the fuel cell stack increase. The tension of one or more fuel cells on the positive end of the stack is a reliable signal for the necessity of replenishing the electrolyte supply.
Additionally according to another beneficial aspect of the invented fuel cell assembly, the electrolyte used to fill the electrolyte reservoir has a composition that differs from the initial composition of the electrolyte in the electrolyte matrixes of the fuel cells in order to compensate disproportionate electrolyte losses during the fuel cell operation. The electrolyte that is used for filling the electrolyte reservoir therefore contains those components that are lost at a higher rate during operation in higher concentrations than loses of the initial or normal composition of the electrolyte in the electrolyte matrixes.
The following describes examples of embodiments of the invented fuel cell assembly based on the drawings: